WUE
Water usage effectiveness: a metric relating water consumption to IT equipment energy.
Also called: Water usage effectiveness
In practice
WUE answers a question PUE cannot: how much water the facility consumes for the computing it supports, usually expressed as litres per kilowatt-hour of IT energy. It exists because cooling strategies that look good on energy can be expensive in water, and a single efficiency number was hiding that trade.
The trade is direct. Evaporative cooling and cooling towers buy thermal performance by evaporating water, which can meaningfully reduce compressor work and improve the energy picture. In a water-stressed region that is not obviously the better choice, and a site can honestly report an improving energy ratio and a worsening water figure from the same change.
As with every metric of this family, the boundary and period decide whether the number means anything. Site water consumption is the common boundary, and a separate accounting sometimes includes the water used to generate the electricity the site consumes, which is a much larger and much less comparable figure. Reports that do not state which they mean are not comparable with each other.
WUE belongs beside PUE, carbon usage effectiveness and the actual service outcome rather than being ranked against them. They have different units, respond to different decisions, and a site optimising any one of them in isolation will eventually damage another.
Scope of this definition
This is an educational summary of how the term is used in data center practice. It is not a standard, a specification or engineering advice, and where a real decision depends on it, the current adopted standards, verified site information and qualified professional review are the correct sources.